Such a coupler is already known in the art, e.g. from the contribution Doc. T1E1.4/94-043 to the ANSI/T1 Telecommunications Committee by John Cook, BT Laboratories (BTL). Therein, the first signal is an Asymmetric Digital Subscriber Line (ADSL) signal, whereas the second signal is a Plain Old Telephone Service (POTS) signal. The second signal means is a POTS receiver/transmitter, whereas the first signal means is an ADSL receiver/transmitter. The first bandpass filter is a highpass filter passing the ADSL signal but blocking the POTS signal. The second bandpass filter is a lowpass filter passing the POTS signal but blocking the ADSL signal.
It is not noted in the latter contribution that part of the first signal is lost via the second bandpass filter when the common line does not sufficiently attenuate the second signal, e.g. when the common line is too short. Indeed, when the second signal is too strong, the filter characteristics of the second bandpass filter change so that part of the first signal is lost via the second bandpass filter. Such a change of filter characteristics of the second band pass filter may occur due to changing characteristics of elements included therein e.g. saturation of an inductor due to a large current or saturation of a capacitor due to a large voltage or saturation of a resistor due to large power dissipation.